1. Field of the Invention
The present invention relates to a color printing technique featuring multiple ink types.
2. Description of the Related Art
Color ink-jet printers are currently used on a wide scale as computer output devices. A common color ink-jet printer features a black (K) ink and multiple types of primary color inks, each having a cyan (C), magenta (M), or yellow (Y) hue. Any color of a color image can be reproduced by these multiple ink types. In particular, a gray color can be reproduced using either a black ink alone or all three types of primary color inks (CMY). The gray color reproduced using the three types of primary color inks (CMY) is commonly referred to as “composite black.”
So-called graininess (image irregularities) is used as an indicator for evaluating image quality. Image graininess is observed when ink dots form separately from each other. Consequently, image graininess is often a problem in comparatively bright image areas with a small number of ink dots.
FIGS. 1(A) and 1(B) illustrate in enlarged form a gray color area reproduced by a black ink alone, and a gray color area reproduced by composite black. The examples shown indicate that when a gray color is reproduced using composite black, the number of ink dots is three times greater than when black ink alone is used. Image graininess commonly increases with a reduction in the number of ink dots. Consequently, gray colors should preferably be reproduced using composite black as much as possible because of considerations related to image graininess.
However, shades other than gray sometimes appear to be present in a gray color reproduced using composite black. This phenomenon is primarily attributed to the spectral characteristics of illuminating light, as described below.
FIGS. 2(A)-2(D) illustrate the principle of color reproduction involving a gray color based on composite black. FIGS. 2(A) and 2(B) depict the spectral distribution PD50(λ) of sunlight and the spectral distribution PA(λ) of an incandescent lamp, respectively. FIG. 2(C) depicts the spectral reflectance RCB(λ) of a gray color reproduced using composite black, and FIG. 2(D) depicts the color-matching functions x(λ), y(λ), and z(λ) of a CIE XYZ color system. As is well known, the tristimulus values X, Y, and Z for expressing the colors of an image area can be obtained by a method in which the product of the spectral distribution P(λ) of illuminating light, the spectral reflectance R(λ) of the image area, and the color-matching functions x(λ), y(λ), and z(λ) is integrated over a visible range of wavelengths λ. In the present specification, the spectral distribution of standard light D50 (CIE) is used as the spectral distribution of sunlight. In addition, the spectral distribution of standard light A (CIE) is used as the spectral distribution of an incandescent lamp.
FIGS. 3(A)-3(D) illustrate the principle of color reproduction involving a gray color based on black ink alone. A comparison of FIGS. 2(C) and 3(C) indicates that the spectral reflectance RCB(λ) of composite black is comparatively irregular and that the spectral reflectance RBK(λ) of the black ink is comparatively smooth. The spectral reflectance of black ink and the spectral reflectance of composite black are noticeably different from each other but appear to have substantially the same color when viewed, for example, in sunlight (standard light D50). The phenomenon in which two colors with mutually different spectral characteristics appear to be substantially the same to the unaided eye under a light source is referred to as “metamerism.”
When viewed in illuminating light other than sunlight (for example, under an incandescent lamp), a gray color printed with composite black sometimes appears to contain colors other than gray. This is because a composite black and a black ink have markedly different spectral reflectance R(λ). Specifically, the tristimulus values (X, Y, Z)D50 obtained using the spectral distribution PD50(λ) of sunlight and the tristimulus values (X, Y, Z)A obtained using the spectral distribution PA(λ) of an incandescent lamp have comparatively large differences when composite black is involved. As a result, a gray color printed with composite black sometimes appears to contain shades or hues other than gray under an incandescent lamp. A black ink, on the other hand, has comparatively smooth spectral reflectance and comparatively similar tristimulus values (X, Y, Z)D50 and (X, Y, Z)DA. As a result, a gray color printed with black ink appears gray even when viewed under an incandescent lamp.
The dependence of the way in which a color is perceived by the unaided eye on the type of illuminating light (as in the case of composite black) is not a particularly desirable feature, creating a need for making a particular color look the same irrespective of the type of illuminating light. In other words, the appearance of a color should not depend on the type of light source used. Gray colors should preferably be reproduced by maximizing the use of black ink in order to reduce the dependence of color appearance on light sources.
In conventional practice, however, not much consideration is given to the dependence of color appearance on light sources during the reproduction of gray colors.